Polycarbonates are a well known class of high impact resistant thermoplastic resins characterized by optical clarity, high ductility as well as other advantageous properties. They are frequently employed as lenses and windows as a result of their transparency. Bisphenol A (BPA) polycarbonate is the predominant commercially available resin of this type. It is derived from 2,2-bis(4-hydroxyphenyl)propane and ordinarily has a glass transition temperature of about 150.degree. C.
It is of increasing interest to prepare polycarbonates which, while retaining the ductility of BPA, have higher glass transition temperatures and are therefore more resistant to softening when heated.
Polycarbonates possessing increased glass transition temperatures are very desirable, for instance, in the automotive and aircraft industries. Particularly, they may be used in the preparation of automotive headlamp lenses which are becoming smaller in size and characterized by closer proximity of the lenses to the heat-generating light source.
The present invention is based on the discovery and method of isolation of substantially pure bisphenols; in particular, 4-[1-[3-(4-hydroxyphenyl)-4-methylcyclohexyl]-1-methylethyl]phenol (hereinafter referred to as BPT-1) and 4,4'-[1-methyl-4-(1-methylethyl)-1,3-cyclohexandiyl]bisphenol (hereinafter referred to as BPT-2) and homologs thereof and homopolycarbonates and copolycarbonates prepared from bisphenols. Substantially pure is defined as at least about 95% pure. The resulting homopolycarbonates possess increased glass transition temperatures on the order of about 198.degree. C. when employing BPT-1 and about 249.degree. C. when employing BPT-2. Moreover, copolycarbonates of BPT-1 and BPA as well as BPT-2 and BPA may be prepared which are also expected to display increased glass transition temperatures.